The present invention relates to an improved process for producing high quality tri-n-butylphosphorotrithioite (phosphite).
Phosphite is the intermediate from which S,S,S-tri-n-butylphosphorotrithioate (useful as a cotton defoliant) is produced. Phosphite has generally been produced by reacting phosphorous trichloride with a substantial excess of butylmercaptan. U.S. Pat. No. 2,682,554, for example, teaches the use of at least 4 mols and preferably 5 mols of butylmercaptan for each mol of phosphorous trichloride. These prior art processes are undesirable because they make it necessary to remove substantial amounts of mercaptan before proceeding with production of the pesticide S,S,S-tributylphosphorotrithioate. Such removal increases production time and is usually incomplete, resulting in a product with undesirably high levels of odorous impurities. These impurities are transferred to the product giving the pesticide an unpleasant odor.
U.S. Pat. No. 2,943,107 teaches that these difficulties may be reduced by decreasing the excess of butylmercaptan from 4 mols to 3 to 3.3 mols per mol of phosphorous trichloride. The butylmercaptan is reacted with phosphorous trichloride until the reaction has reached 85-99% completion. An oxidizing agent is then added to oxidize the phosphite to the desired product. The oxidizing agent described as preferred is air. However, the highly odorous dibutyldisulfide is one of the by-products formed from the unreacted butylmercaptan present in the reaction mixture oxidized by air.
In one attempt to overcome these odor problems, phosphorous trichloride and a slight excess (i.e., 5%) of butylmercaptan were reacted in an inert atmosphere. The hydrogen chloride which formed during this reaction was subsequently removed by a nitrogen purge at elevated temperature. The phosphite thus prepared was oxidized with hydrogen peroxide at a moderate temperature and then stripped with steam to remove the volatiles.
Each of the processes discussed above has the disadvantage of requiring the presence of large quantities of hazardous phosphorous trichloride be present in the reactor initially. Expensive equipment capable of withstanding a highly corrosive atmosphere, extensive safety measures and safety equipment were required to produce phosphite by these known processes.
It would be advantageous to have a process for producing phosphite which did not involve the presence of large quantities of phosphorous trichloride in the reaction at any stage of the process for safety considerations and which also did not produce an odorous product.